Increasing O2 has physiological effects different from the effects of decreasing CO2. It is not just the ratio or difference between the concentrations that matter.
For example, a major effect of increasing CO2 is to make the blood more acidic (or to put more stress on the homeostatic mechanisms that maintain a target pH), and my guess is that no amount of increasing O2 will cancel that out.
Mostly systems programming. In particular, he wrote its graphics system, then rewrote it, then rewrote it again. He led Plan 9 for many years. So, he has plenty of experience in systems programming.
What he seems to lack IMHO is sufficient curiosity about computing innovations not produced by him or the prestigious institutions he has worked for (Bell Labs, Google).
For example, a major effect of increasing CO2 is to make the blood more acidic (or to put more stress on the homeostatic mechanisms that maintain a target pH), and my guess is that no amount of increasing O2 will cancel that out.